1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device in which a bipolar transistor and a MOS transistor are integrated on the same substrate.
2. Related Art
MOS transistors have many advantages but, on the other hand, have disadvantages such as low mutual conductance, a limited switching speed when used as an element of the integrated circuit for driving capacitive load, and limited performance as a power semiconductor device. The advantages and disadvantages of bipolar transistors are opposite to those of MOS transistors. For providing the advantages of the both, composite elements in which a bipolar transistor and a MOS transistor are integrated in a monolithic manner have been used.
A semiconductor device comprising those composite elements aims at high-speed operation of the device. For this purpose, a thick insulating film is formed under the emitter electrode in order to decrease the parasitic capacitance of the bipolar transistor and, for the MOS transistor, a thin gate insulating film is formed to provide improved performance.
For this reason, the MOS transistor having a gate insulating film typically 10 nm or less in thickness is formed, and thereafter an insulating film as thick as 50 to 200 nm is formed to integrate the bipolar transistor.
Incidentally, in the case where the gate of the MOS transistor to be formed is, for example, approximately 0.3 .mu.m or less in length, heat treatment needs to be reduced in order to prevent diffusion of the dopant of which the source/drain is composed. On the contrary, the bipolar transistor to be formed following the MOS transistor is designed to obtain a desired current amplification factor by allowing the dopant to diffuse to a predetermined region and thus forming the emitter.
Therefore, forming the MOS transistor first would cause the performance of the bipolar transistor to be degraded because the bipolar transistor could not be formed by performing heat treatment at high temperatures for a long duration in order to ensure the characteristics of the MOS transistor. On the other hand, heat treatment performed at high temperatures for a long duration in order to ensure the performance of the bipolar transistor would degrade the performance of the MOS transistor.